The conventional thermal and catalytic Claus process for sulfur recovery from hydrogen sulfide (H.sub.2 S) containing gas streams is widely practiced and accounts for a major portion of total production of sulfur. In a Claus process sulfur recovery unit, the first step is the thermal oxidation of a fraction of H.sub.2 S in the acid gas feed to sulfur dioxide (SO.sub.2). This is typically carried out in the thermal reaction zone (Claus furnace) by the addition of air to acid gas which contains mainly H.sub.2 S but may also contain carbon dioxide (CO.sub.2), and possibly hydrocarbons. In this oxidation step, in addition to SO.sub.2, the products also include elemental sulfur and water. The Claus reaction, shown below, can then be continued in downstream catalytic reactors having an effective Claus catalyst. The conversion achieved in each reactor is equilibrium limited. Typical reactions occurring in such sulfur recovery processes can be summarized by the following equations: EQU H.sub.2 S+3/2 O.sub.2 .revreaction.H.sub.2 0+SO.sub.2 (Oxidation) (1) EQU 2H.sub.2 S+SO.sub.2 .revreaction.2H.sub.2 O+3S (Claus Reaction) (2) EQU H.sub.2 S+1/2 O2.revreaction.H.sub.2 O+S (Overall) (3)
The use of air as the source of elemental or free oxygen in the thermal reactor introduces large quantities of nitrogen (N.sub.2) into the process. Equipment must be sized to handle this volume of inert gas. By increasing the concentration of oxygen in the oxygen source, the quantity of nitrogen which must be processed can be reduced. Such oxygen enrichment can significantly increase the capacity of an existing sulfur recovery unit and/or reduce the capital investment for a new unit.
The acid gas feed to the sulfur recovery unit may come from a variety of sources. Some of these acid gas streams are almost pure hydrogen sulfide (on a dry basis), but others have a high concentration of inert components, which can be carbon dioxide, for example. If the ratio of inert component (such as carbon dioxide) to hydrogen sulfide is greater than about 1/1, then the flame temperature in the thermal reaction zone is reduced and approaches a zone of unstable combustion. Special design methods have been used to handle these acid gas streams, such as the split flow sulfur recovery process, but this can cause operating problems and is not satisfactory for very high ratios of carbon dioxide/hydrogen sulfide, such as 5/1 or higher. An ideal solution is to use enriched air or a high concentration oxygen stream instead of air. This solves the problem of stabilizing the flame because eliminating or substantially reducing the nitrogen rate to the thermal reaction zone causes an increase in temperature that stabilizes the flame. In this case, the flame temperature in the thermal reaction zone will be satisfactory but the temperature rise in the downstream catalytic reactors may be too high; however, this problem can be solved by the process of this invention. Further, with either a low CO.sub.2 acid gas or a high CO.sub.2 acid gas, if industrial grade oxygen or enriched air is used instead of air, the nitrogen can be substantially eliminated from the feed gas to the furnace and, therefore, substantially eliminated from the tail gas. This can reduce the amount of tail gas and make it possible to increase conversion to sulfur and also to reduce emissions. However, there are problems caused by using pure oxygen, such as a very high temperature rise in the thermal reactor or reaction furnace, and a very high temperature rise in the downstream catalytic reactors. Such temperature rise can exceed the metallurgical limits of conventional thermal reactors and can be of such an extent as to damage the downstream catalytic reactors.
In the past this type of process has not been attractive economically in some locations because of the high cost or nonavailability of the oxygen or enriched air stream. With the advent of nitrogen injection facilities for use in some oil reservoirs, for example, there are now a larger number of locations where a stream of enriched oxygen in air may be made available for beneficial use in this process. Other sources of pure oxygen can of course also be used.
Giech, U.S. Pat. No. 4,138,473 (1979) deals with a sulfur recovery process using pure oxygen for combusting the feed gas rather than air as in conventional Claus processing. A mixture of H.sub.2 S and SO.sub.2 is thus produced, which is reacted successively over a series of catalytic converter beds wherein they react to produce water and elemental sulfur, the elemental sulfur being condensed after each converter, and the gaseous output of each converter being repressurized and reheated before entering the next successive converter to improve the yield of sulfur therein. The gaseous output of the final converter of the series is combusted with oxygen in a final catalytic converter to convert any remaining H.sub.2 S to SO.sub.2. The H.sub.2 O, SO.sub.2 and CO.sub.2 mixture emerging from said converter is treated to remove therefrom as separate streams the water, the CO.sub.2, and the SO.sub.2. A concentrated SO.sub.2 stream is returned to the beginning step of the process. The SO.sub.2 and water are separated from the CO.sub.2 by contacting the combined stream with cold water. The CO.sub.2 which is only partly soluble in water is removed overhead. The cold water containing absorbed water and SO.sub.2 is sent to a stripper where it is heated to remove the SO.sub.2 SO.sub.2 stripped from the water, together with steam, flows from the stripper to a condenser where steam is condensed and the SO.sub.2 is returned to the beginning step of the process. In this way, pure oxygen can be used to combust the acid gas; however, Giech is silent as to the problem of moderating the temperature rise in the combustion zone when pure oxygen is used and also in downstream catalytic converters. Furthermore, this process requires a large amount of water to absorb the SO.sub.2 and also requires heat input in the SO.sub.2 stripper sufficient to drive the SO.sub.2 overhead.
Accordingly, it is desirable to achieve the advantages of operation with oxygen enriched air or substantially pure oxygen in a Claus plant thermal reaction zone while avoiding the undesirable temperature rise which can result from such use.